Thermal expansion and cation disorder in Bi2InNbO7

Local crystal chemistry, structured diffuse scattering and the dielectric properties of (Bi1−xYx)2(MIIINbV)O7 (M=Fe3+, In3+) Bi-pyrochlores

“…In a recent diffraction investigation of Bi 2 InNbO 7 [14], we reported preliminary electron diffraction evidence for b-cristobalite-type orientational disorder (see Fig. 2) of the O 0 Bi 4 tetrahedra that make up the O 0 A 2 sub-structure of the ideal pyrochlore structure type.…”

“…1), many are known to be strongly displacively disordered, particularly on the O 0 A 2 substructure (see e.g., [10][11][12][13][14][15] and references contained therein). Indeed, after careful consideration, Vanderah et al [15] have recently even gone so far as to conclude that ''y the occurrence of displacive disorder in the O 0 A 2 network is required for the formation of pyrochlore y'' in many such Bi-pyrochlore systems.…”

“…Indeed, after careful consideration, Vanderah et al [15] have recently even gone so far as to conclude that ''y the occurrence of displacive disorder in the O 0 A 2 network is required for the formation of pyrochlore y'' in many such Bi-pyrochlore systems. It has frequently been found, for example, that Bi ions when placed on the 16d ideal A site positions of the pyrochlore structure type exhibit anomalously large atomic displacement parameters (adp's) [10][11][12][13][14]. Indeed the adp's have often been so large that the structure has been found to be rather better modelled by displacing the Bi/A site ions perpendicular to the local three-fold O 0 -Bi-O 0 axes by up to $0.5 Å or so into 96g or 96h sites (see, e.g., Fig.…”

“…1 of [12] and Refs. [10][11][12][13][14][15]). Formally, the Bi and other A site ions are then able to occupy any one of up to 6 equivalent such displacively disordered local positions (see, e.g., Fig.…”

First-principles calculation of the structure and dielectric properties ofBi2Ti2O7

“…Both experimental and theoretical studies of Bi 2 Ti 2 O 6 OЈ ͑Refs. 2, 4, and 5͒ and other bismuth pyrochlores 5,[8][9][10][11][12][13][14] indicate that the Bi 2 OЈ network is distorted compared to the network in the ideal pyrochlore structure. Similarities between the structure of the Bi 2 OЈ network in Bi 2 Ti 2 O 6 OЈ and the SiO 2 network in ␤ cristobalite have been highlighted.…”